Brushless DC motors are an accepted staple of the electronics industry. This type of motor is commonly used in applications which require extended continuous high speed operation. Brushless DC motors are widely used in the data processing industry as drive motors in rotating media drives and in other applications.
The construction of brushless DC motors has long been known in the industry. These motors are constructed in many different phases and excitation modes. Two of the most common forms of brushless DC motors in the prior art are the three phase bipolar excitation type of motor and the two phase unipolar excitation type. An example of a three phase motor, exemplifying one of these types, is found in a patent entitled "Brushless DC Motor Assembly With Improved Stator Pole", U.S. Pat. No. 4,499,407 issued to Macleod.
Other examples of brushless DC motors incorporating conventional structures and associated circuitry may be found in U.S. Pat. No. 4,430,603 issued to Muller and U.S. Pat. No. 4,376,261 issued to von der Heide et al.
The three phase bipolar version the brushless DC motor is popular since it provides high performance characteristics. One of the most important and appreciated characteristics of the three phase bipolar type is the fact that it does not suffer greatly from a phenomenon known as torque ripple. Torque ripple is defined as the presence of positions in the rotation of a motor wherein there is either zero or very low torque in relation to the motor's peak torque. However, the three phase bipolar motor is disfavored in cost or space sensitive applications because, in addition to the fact that it requires relatively complex commutation logic and circuitry, this type of motor requires a minimum of six switching devices to achieve continuous motor rotation.
The two phase unipolar excitation type motor, an example of which may be found in U.S. Pat. No. 4,429,263, issued to Muller, is one of the simplest forms of operable motors. This type of motor requires two separate sets of stator windings and two switching devices, plus a single position sensor and commutation logic and circuitry. The two phase unipolar type of motor has, in prior art configurations, suffered from pronounced torque ripple characteristics. One of the undesirable effects of torque ripple is that the motor starting torque is very low relative to the running torque. This has posed a particular problem for makers of data disk drives in that high starting torque is desirable, but running torque is intentionally limited.
Further, most prior art designs of the two phase unipolar brushless DC motor have not been inherently self starting. There are positions in the rotational axis of these motors from which either zero or minimal torque is produced, and from which the motor cannot generate sufficient torque to begin rotation. These positions are known as detent positions. Solutions to this problem in the prior art have involved schemes to prevent or discourage the motor from coming to rest in one of these detent positions. An example of this type of motor can be seen in the U.S. Pat. No. 4,499,407 issued to Macleod.
The Macleod invention utilizes an intentionally "out of phase" torque to eliminate zero torque positions. This high out of phase torque (cogging torque) is undesirable in operating modes other than start up, as the magnitude of this cogging torque is invariant with speed, thus causing high speed torque ripple and vibration in the application.
No prior art method is presently known for providing a two phase unipolar excitation brushless DC motor which can produce a significant portion of its maximum torque from any position in its rotation; while maintaining relatively low levels of cogging torque.